Earth Science Ch. 20: A Trip Through Geologic Time

Earth Science Ch. 20:A Trip Through Geologic Time

Ch. 20-1: The Geologic Time Scale

Key Terms• Precambrian Era

• Paleozoic Era

• Mesozoic Era

• Cenozoic Era

California Content Standards for Earth Science Addressed:

Earth Science1(b) Students know the evidence from Earth and moon rocks indicates that the solar system was formed from a nebular cloud of dust and gas approximately 4.6 billion years ago. 1(c) Students know the evidence from geological studies of Earth and other planets suggest that the early Earth was very different from Earth today. 1(f) Students know the evidence for the dramatic effects that asteroid impacts have had in shaping the surface of planets and their moons and in mass extinctions of life on Earth.

Ch 20-1:Why Do I Need To Know This?

1. So that you can understand how and why life has developed and evolved over time.

2. So that you can know and understand the scientific evidence for evolution.

3. So that you can understand the key events that have shaped life on this planet and what may happen to life in the future.

Key Sections Ch. 20-1

• The Geologic Time Scale

• The Nebular Theory

The Geologic Time Scale

• Earth history has been divided into 4 major eras—each with a different amount of time.

• The eras are based on the types of fossils found in the rocks.

• The eras are sub-divided into smaller units of time called periods.

The Nebular Theory• According to the Nebular

Theory, the Earth formed from the remains of a star that exploded about 7 billion years ago.

• The dust and gases from that explosion were slowly pulled together by the force of gravity. The clumps of dust and gas grew over time.

• As gravity pulled the dust and gases together, they began to rotate. The heavier gases and dust went towards the center and the lighter gases went to the outside edge.

• Eventually the sun and the planets formed. When the sun became massive enough, it began to fuse and emit light.

Ch. 20-2: Early Earth History

Key Terms• Trilobite


Earth Science1(b) Students know the evidence from Earth and moon rocks indicates that the solar system was formed from a nebular cloud of dust and gas approximately 4.6 billion years ago. 1(c) Students know the evidence from geological studies of Earth and other planets suggest that the early Earth was very different from Earth today. 1(f) Students know the evidence for the dramatic effects that asteroid impacts have had in shaping the surface of planets and their moons and in mass extinctions of life on Earth. 8(a) Students know the thermal structure and chemical composition of the atmosphere. 8(b) Students know how the composition of Earth's atmosphere has evolved over geologic time and know the effect of outgassing, the variations of carbon dioxide concentration, and the origin of atmospheric oxygen.

Ch. 20-2:Why Do I Need To Know This?

1. So that you can know how the Earth was formed.

2. Because the Earth at its beginning was VERY different than it is today.

3. Because you may invent a time machine and will need to know how to survive in the past!


• Early Earth History

• The Precambrian Era: The Dawn of Life

• The Paleozoic Era: Life Comes Ashore

Early Earth History

• The vast majority of Earth history occurs during the “early” Earth history.

• During this time period the following major events happen:

1. The planet forms;

2. The initial atmosphere forms;

3. The atmosphere changes to include oxygen;

4. Life develops;

5. Life evolves from single-celled organisms to jellyfish to fish and sharks, to life on the land.

The Precambrian Era: The Dawn of Life

• The Precambrian Era is the longest lasting geological era, lasting over 4 billion years!

• Early life most likely developed in the oceans and used simple chemicals in the ocean water and energy from the sun to live.

• Scientific studies have shown that the basic building blocks for life can form from the mixture of gases and chemicals that existed in the early Earth history. Life in the Precambrian Era developed quickly, appearing approximately 300 million years after the planet formed.

The Precambrian Era: The Dawn of Life

• Blue-green algae and stromatolites are important life forms from the early Precambrian. They breathed in nitrogen and exhaled oxygen which made life for us possible.

• Without these changes, life as we know it would not be possible.

• By the end of the Precambrian Era, life had evolved to include jellyfish, worms, plants, sponges and corals.

The Paleozoic Era: Life Comes Ashore

• The Paleozoic Era (ancient time) lasts from about 570 million years ago until about 225 million years ago.

• There are many periods in the Paleozoic Era which are named for the places where the rocks and fossils were found.

• The Paleozoic Era is often called the “Age of Fishes” because fishes, sharks, and shellfish evolve during this time period.

• Fishes are the first organisms to have vertebrates, which are spines.

The Paleozoic Era: Life Comes Ashore

• Other key organisms from this time period are Trilobites, which are insects ranging in size from an inch to over 2 feet long!

• After fishes evolve, plants start to appear on the land, followed by amphibians, which were the first animals on the land.

• At the end of the Paleozoic Era, though, about 99% of all life forms were wiped out in a major disaster, most likely a meteor impact.

• And, at the end of the Paleozoic Era, the Earth’s climate becomes much warmer, leading to a world-wide drought.

Ch. 20-3: Middle and Recent Earth History

Key Terms• Triassic Period

• Jurassic Period

• Cretaceous Period


Earth Science

1(f) Students know the evidence for the dramatic effects that asteroid impacts have had in shaping the surface of planets and their moons and in mass extinctions of life on Earth.


1. Because dinosaurs are cool (ask me about copperlites)!

2. Because we are mammals, and during this time period mammals begin to evolve (although mammals were MUCH smaller back then).

3. Because the dinosaurs were wiped out in a cataclysmic event, which could happen again in our lifetimes!


• Middle and Recent Earth History

• The Mesozoic Era: Mammals Develop– The Triassic Period– The Jurassic Period– The Cretaceous Period

• The Cenozoic Era: A World With People– The Tertiary People– The Quarternary Period

Middle and Recent Earth History

• There are 4 major events that happen during middle Earth history. They are the:

1. Formation of Pangea;

2. Evolution of mammals;

3. Breakup of Pangea;

4. Mass extinction of the dinosaurs.

The Mesozoic Era: Mammals Develop

• Middle Earth history is called the Mesozoic Era (middle time).

• The Mesozoic Era lasts from 225 million years ago to 65 million years ago.

• The Mesozoic Era is divided into 3 periods which are the:

1. Triassic;2. Jurassic;3. Cretaceous.

The Triassic Period• The Triassic Period lasts from 225 to

190 million years ago.• At the beginning of the Triassic, the

world-wide drought is still present, and in fact, the climate is getting hotter.– This has to do with Pangea and

the distribution of heat throughout the planet.

• However, during the Triassic, Pangea begins to split apart and the Atlantic ocean begins to form.– As North America splits away

from North Africa, the Appalachian Mountains begin to form.

• Reptiles move back into the water and mammals begin to evolve.

The Jurassic Period

• The Jurassic Period lasts from 190 to 136 million years ago.

• The Jurassic Period is knows as the “Age of Dinosaurs” because dinosaurs become the dominant life form.

• The Rocky Mountains and Sierra Nevada Mountains begin to form and the Appalachian Mountains erode.

The Cretaceous Period• The Cretaceous Period lasts from 136

million years ago to 65 million years ago.• Most of the continents become flooded

with great seas on the land.• Dinosaurs are still the dominant life form,

but mammals still exist and flowering plants begin to evolve.

• During the Cretaceous Periods, South America splits apart from Africa, and North America slams into Europe recreating the Appalachian Mountains.

• By the end of the Cretaceous Period, almost all the life on the planet gets wiped out in a massive event (within about a 10 year period). – Most scientists believe that the

extinctions had to do with a giant meteor that hit the Earth, most likely in the Yucatan peninsula.

The Cretaceous Period• There is a lot of evidence that

the dinosaurs were wiped out by a giant meteorite that hit the Earth about 65 million years ago.

• Some of this evidence includes:

1. The rapid disappearance of all dinosaur fossils;

2. A layer of dirt found around the entire earth containing isotopes common in meteorites;

3. Evidence of a giant meteor impact that hit the Earth in the Yucatan Peninsula about 65 million years ago.

The Cenozoic Era: A World With People

• The Cenozoic Era lasts from 65 million years ago until today, and is a time of great change for the Earth.

• The plates continue to drift apart.• The climate cools down

dramatically, creating great sheets of ice across most of the continents.

• Mammals and flowering plants become the dominant life forms on the planet.

• Eventually, humans begin to evolve and walk upright!

The Tertiary Period• The Tertiary Period lasts from 65

million to 1.8 million years ago. • During the Tertiary Period, the

Earth begins to cool down.• Throughout the Tertiary Period,

Africa and Europe move away from North and South America.

• Grasses evolve as do many mammals that feed on them.

• Primates evolve from the size of a mouse to about 3.5 feet tall. Eventually, one branch of the primate tree becomes the hominid line, from which we evolve.

The Quarternary Period

• The Quarternary Period lasts from 1.8 million years ago until today!

• The Quarternary Period is marked by intervals of glaciation which last about 100,000 years, separated by periods of warmth that last about 20,000 years.

• Throughout this time period, homids continue to evolve, eventually leading to homo sapiens.

• The last period of glaciers ended about 18,000 years ago. At that time, the ice was over 5 km thick in Minnesota!

• The end of the last ice age is marked by another major period of extinction, most likely due to human activities.

Ch. 20-4: Evolution: Change Over Time

Key Term• Evolution

• Adaptation

• Species

• Genes

• Punctuated Equilibrium


Life Science7 The frequency of an allele in a gene pool of a population depends on many factors and may be stable or unstable over time. As a basis for understanding this concept: 7(c) Students know new mutations are constantly being generated in a gene pool. 7(d) Students know variation within a species increases the likelihood that at least some members of a species will survive under changed environmental conditions.


Life Science8 Evolution is the result of genetic changes that occur in constantly changing environments. As a basis for understanding this concept: 8(a) Students know how natural selection determines the differential survival of groups of organisms. 8(b) Students know a great diversity of species increases the chance that at least some organisms survive major changes in the environment. 8(c) Students know the effects of genetic drift on the diversity of organisms in a population. 8(d) Students know reproductive or geographic isolation affects speciation. 8(e) Students know how to analyze fossil evidence with regard to biological diversity, episodic speciation, and mass extinction.


1. Because evolution has happened and is continuing to happen.

2. Because scientists are using our understanding of genetics to cure diseases and cure patients.

3. So that you know that humans did not evolve from monkeys. Rather, humans and monkeys evolved from common ancestors.


• Evolution: Change Over Time

• Natural Selection

• Genetics

• Mutations: Agents of Change

• Punctuated Equilibrium

• The Fossil Record

• Human Evolution

Evolution: Change Over Time• Evolution is defined as the

change in a species over time.• A species is a group of

organisms that can interbreed with each other and produce fertile offspring with the same basic characteristics.

• Three major scientists who developed key scientific understandings for genetics are:

1. Charles Darwin

2. Gregor Mendel

3. Stephen Jay Gould

Natural Selection• Charles Darwin was a British

biologist who lived from 1809 to 1882.

• In 1831 Darwin set sail around the world on a ship called the H.M.S. Beagle to study plants and animals.

• In 1832, the Beagle stopped in a group of very remote islands called the Galapagos Islands (off of South America).

• Darwin studied many of the birds there, particularly a small bird called the Finch.

Natural Selection• Finches are rather common small birds which cannot fly

back and forth to South America.• Darwin noticed that there were 13 different types of finches

on all of the Galapagos Islands, but only 1 or 2 types on each individual island.

• And, Darwin noticed that the different types of finches had profoundly different beaks.

• He realized that the type of beak that occurred on each island related to the type of food that was available on the island.

Natural Selection• Darwin realized that at one point a group of finches

came to the Galapagos Islands. Some of the islands had trees with nuts, others did not.

• In that group of original finches, some finches had shorter beaks and some had longer beaks.

• Those that had shorter beaks were able to survive on the islands with trees with seeds, whereas those with longer beaks were able to survive on islands with insects.

• Over time, the finches that had the evolutionary advantages survived, whereas those with the disadvantage died out.

• Eventually, the populations on the island evolved to have only 1 or 2 species on each island, depending on the food availability and beak structure.

Natural Selection• Darwin published his Theory of Natural Selection

in a book called On Origins of the Species.• The Theory of Natural Selection states that:

1. Within every species there is variation amongst the members (some are taller/shorter/etc.).

2. In every ecosystem, some traits will be favored and some will not be favored.

3. As a result, over time, those members with the traits that are favored are more likely to survive and have offspring, leading to their favorable traits becoming more common within the species.

4. Eventually, if a trait becomes common to all members of a species, the species has evolved.

Genetics• Gregor Mendel (1822-1884)

was a Czeck monk/botanist who raised pea pods.

• He discovered that some pea pods had different colors and shapes.

• As Mendel bred the pea pods, he discovered that certain traits were passed onto the offspring and others were not.

• Mendel’s research led to the Punnett Square Diagram, making him the “father” of genetics!

Genetics• All of life is controlled by DNA.• An organism gets half of its DNA from its mother

and half of its DNA from its father.• While DNA is a very long molecule, portions of it

move together as genes.• There are 2 types of genes: Dominant genes

and Recessive genes.• Dominant genes are the genes that we will

actually see in an organism.• Recessive genes are not seen in an organism,

but can be passed onto the children.• In the Punnett Square, a dominant gene is

represented as a capital letter. A recessive gene is represented with a lower case letter.

GeneticsThe Punnett Square

Mutations: Agents of Change• All cells reproduce inside a body to create

new tissues.• In order to create a new cell, a parent cell

copies its DNA and creates a new set of DNA for the new cell which then splits off from the parent cell. This is called mitosis.

• During sexual reproduction, the egg and sperm cells undergo an extra step in which they split the DNA between the sex cells. This is called meiosis.

• This splitting is what gives a sex cell its dominant and recessive genes.

Mutations: Agents of Change

Mutations: Agents of Change

• Sometimes, during the copying stage of cell division, the DNA is copied incorrectly.

• This incorrect copy of the DNA is called a mutation.

• If the mutation occurs during mitosis (normal cell splitting) only that organism can be affected by the mutation (it cannot pass that mutation onto its children).

• Some cancers are examples of mutations that can occur during mitosis.

Mutations: Agents of Change• If the mutation happens during meiosis (creation of

sex cells) then the offspring WILL inherit the mutation and will be able to pass the mutation onto its offspring as well.

• Most mutations (about 99%) do not result in a live birth (the mother’s immune system detects it and destroys it or the mutation is too severe to survive).

• Those that do produce live offspring often do not have any benefit whatsoever and are called benign.

• However, over time, multiple mutations can occur within the species that could produce an evolutionary advantage.

• At some point, if enough mutations occur and get spread throughout the population, a new species will evolve!

Punctuated Equilibrium • Stephen Jay Gould (1941 – 2004) was a

geologist/biologist who taught at Harvard.• He noticed that evolution typically happened when

populations were isolated and undergoing environmental stress (such as a changing climate).

• Based on his research, he developed the Theory of Punctuated Equilibrium.

• According to punctuated equilibrium, a species achieves stability (no mutations) for long periods of time.

• However, when the environment changes, and populations of the species become isolated, the populations will either become extinct or will undergo massive amounts of mutation very quickly, most likely resulting in the rise of a new species.

Punctuated Equilibrium

The Fossil Record

• The fossil record shows definite signs of evolution.

• During the Precambrian Era, only single celled organisms lived.

• As time passed, life became more complex.

• Organisms that evolved from common ancestors show common characteristics in their bones.

The Fossil Record• While there are gaps in the

fossil record, that has to do with how and why fossils form.

• The chances of becoming fossilized are very small (less than 1 in 10 million!). And, the chances of humans finding any specific fossil are even smaller than that! So, humans are not likely to ever find fossils from every species that ever lived on the planet.

• Most fossils form in shallow water environments. So, it makes sense that we have more fish/water animal type fossils than other types of fossils

The Fossil Record Plate Tectonics and Evolution

• We can see how plate tectonics has caused major changes in evolution.

• Primates exist in both South America and Africa.– Primates in Africa do not

have prehensile tails (tails that they can use for hanging from trees).

– Primates in South America do have prehensile tails.

– There also are differences in the tooth structure between African and South American primates.

• These differences are a result of the two groups of primates being separated by the Atlantic Ocean!

Human Evolution• Humans and the great apes evolved from a common

ancestor approximately 5.5 million years ago.• By about 3.4 million years ago, our ancestors, called

australopithicus, had evolved the ability to walk upright. Unlike humans, most great apes walk on their knuckles. so we had clearly stopped evolving in common with great apes by that time!

• By 2.4 million years ago, humans had become their own separate line on the hominid chain and were called Homo habilis.– Homo habilis was able to use tools!

• By about 1.4 million years ago Homo habilis had become extinct and was replaced by Homo erectus.– Homo erectus was able to use fire!

• By about 300,000 years ago, humans had evolved into Neanderthals.

• By about 35,000 years ago, humans had evolved into Homo sapiens.

• Where humans will go from here is anyone’s guess!

Human Evolution

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Earth Science Ch. 20: A Trip Through Geologic Time